1. Field of the Invention
This invention relates to a light transmission type solar cell, particularly relates to a solar cell of high transparency comprising a light transmission type metal electrode. Further, the present invention also relates to a method for producing that solar cell.
2. Background Art
Solar light energy shining onto the whole earth is said to be a hundred thousand times as large as the electric power that the whole word consumes, and hence we are already surrounded with a huge energy resource even if especial industrial activities are not performed. In order to effectively exploit this huge energy resource, there have hitherto been developed techniques for solar cells utilizing the solar light energy. Solar cells convert the solar light energy into electric energy, which is convenient for human beings to use, and are therefore regarded as important key devices for solving the energy exhaustion problem discussed recently.
Meanwhile, solar cells manufactured at present are categorized into two types, namely, silicon (Si) type and compound semiconductor type. The silicon type solar cells are further categorized, depending on the constitution of silicon layers formed therein, into four types such as single crystal silicon type, poly-silicon type, amorphous silicon type, and crystallite silicon type. According to the structures thereof, they are also categorized into laminate type (tandem type), thin-film silicon type, hybrid type and others. Those kinds of silicon type solar cells thus categorized are different from each other in various aspects such as conversion efficiency, cost and processing performance, and hence are properly selected depending on the use, setting place and the like.
From the viewpoints of cleanness and safety, solar cells as sustainable energy sources are one of the notable objects of studies still drawing attention of people in 21st century. For example, there are studied solar cells that have additional values such as flexibility in addition to the basic function of generating electricity when exposed to sunshine. Besides the flexible solar cells, examples of those additional-valued solar cells also include a light transmission type solar cell, which generates electricity from sunshine and simultaneously which transmits solar rays. This type of solar cell is also referred to as “see-through solar cell”. Since this type solar cell itself is semi-transparent, it not only generates electricity but also transmits sunlight moderately when set on a window or the like of a building. The light transmission type solar cell is therefore expected to have effects of reducing the amount of direct sunlight into rooms and of improving cooling efficiency in buildings.
On the other hand, however, since solar cells are fundamentally devices for absorbing and converting light into electric energy, they preferably absorb a large amount of light. Accordingly, in view of power generation efficiency, solar cells are preferably not transparent and hence generally they hardly transmit light. For this reason, in conventional techniques, a module including a solar cell is partly subjected to laser scribing or punching to form openings or otherwise plural solar cells in a module are arranged at intervals in order that light can penetrate through the openings or intervals and thereby that the module as a whole can be transparent. However, there is evidently a trade-off relation between the opening ratio (area ratio of the openings to the light incident surface) and the power generation efficiency. In fact, a practical solar module can have an opening ratio of 10% to 20% at the most. This opening ratio, however, provides such insufficient transparency that the module obviously looks dark. There is, therefore, room for improvement in lighting.
To cope with the above problem, there is proposed a solar cell transparent itself. For producing that solar cell, both a light-incident side electrode layer and a counter electrode layer, between which a photoelectric conversion layer is placed, are normally made of transparent metal oxide such as ITO (indium tin oxide) without forming the openings or intervals. In JP-A 1990-312285 (KOKAI), for example, an ITO electrode layer is formed on each of the top and bottom surfaces of an amorphous silicon layer to produce a light transmission type solar cell. However, a transparent electrode layer made of an oxide such as ITO generally has a smaller electroconductivity than a metal electrode by two orders of magnitude. Accordingly, in order that the oxide electrode layer can have an electroconductivity comparable to that of the metal electrode, it is inevitably necessary to thicken the oxide layer considerably. However, the thickened oxide layer generally gives a photoelectric conversion efficiency as small as 1% or less, and hence further studies are needed for putting the oxide layer to practical use. In order to improve this problem, JP-A 1993-14096 (KOKAI) proposes a solar cell in which an electrode on the light-receiving side of an amorphous silicon layer is made of ITO but in which the other electrode on the opposite side has a laminate structure comprising a thin semi-transparent metal layer and a transparent electroconductive metal oxide layer. This solar cell has a conversion efficiency of approx. 7 to 8%, and hence is improved in conversion efficiency as compared with the above conventional solar cell. However, from the viewpoint of transparency, the proposed solar cell has a light-transmittance as small as approx. 10% at the wavelength of 700 nm although having a relatively large transmittance of approx. 28% at the wavelength of 800 nm. Accordingly, there is still room for improvement in light-transparency in the visible range although that solar cell has improved light-transparency in the infrared range, in which human eyes have low sensitivity to light.
As described above, it still cannot be clearly said that the light transmission type solar cell makes a breakthrough in the trade-off relation between the light-transmission and the power generation efficiency. It is, therefore, desired to provide a novel solar cell technique capable of making a breakthrough in the trade-off relation between the opening ratio and the power generation ability.